It is known in the art to provide for low compression ratio diesel combustion systems, primarily to minimize emissions of soot and nitrogen oxides (NOx). Low compression ratios, however, are most beneficial during warm engine operation, warm ambient conditions and high engine loads and speeds, but may result in poor combustion quality and excessive hydrocarbon emissions during relatively cool engine operation, cool ambient conditions and at low loads and speeds. It is known in the art to be useful to provide means for varying the compression ratio of a diesel engine to tailor the engine to operating conditions to thereby provide high compression ratios and low compression ratios at optimal times in the engine's operation. Many devices have been proposed for compression ratio variation, including variable valve actuation mechanisms and engine components such as pistons and cylinder heads with movable combustion chamber walls. In general these devices are relatively complex and add significant cost to the manufacture of an engine.
In spark ignition engines, cam phasers are known as simple devices for varying cam timing and thus changing valve timing to the extent permitted by the camshaft layout. These devices normally provide for advancing or retarding the cam timing in order to obtain desirable operating characteristics. It is believed that one of the reasons cam phasers have not been utilized on diesel engines is because the piston to cylinder head clearance is so small that altering intake and exhaust valve timing may result in contact of the pistons with the valves. A simple and relatively low cost apparatus and method for controlling in-cylinder temperature in a diesel engine is desired.